This invention relates to cellular communication systems, and, more particularly, to an apparatus for use by mobile subscriber units to provide directional transmitting and receiving capabilities.
The bulk of existing cellular antenna technology belongs to a low- to medium-gain omni-directional class. An example of a unidirectional antenna is the Yagi antenna shown in FIG. 1. The Yagi antenna 100 includes reflective antenna elements 105, active antenna element 110, and transmissive antenna elements 115. During operation, both the reflective and transmissive antenna elements 105, 115, respectively, are electromagnetically coupled to the active antenna element 110. Both the reflective antenna elements 105 and the transmissive antenna elements 115 re-radiate the electromagnetic energy radiating from the active antenna element 110.
Because the reflective antenna elements 105 are longer than the active antenna element 110 and spaced appropriately from the active antenna element 110, the reflective antenna elements 105 serve as an electromagnetic reflector, causing the radiation from the active antenna element 110 to be directed in the antenna beam direction 120, as indicated. Because the transmissive antenna elements 115 are shorter than the active antenna element 110 and spaced appropriately from the active antenna element 110, electromagnetic radiation is allowed to propagate (i.e., transmit) past them. Due to its size, the Yagi antenna 100 is typically found on large structures and is unsuitable for mobile systems.
For use with mobile systems, more advanced antenna technology types provide directive gain with electronic scanning, rather than being fixed, as in the case of the Yagi antenna 100. However, the existing electronics scan technologies are plagued with excessive loss and high cost, contrary to what the mobile cellular technology requires.
Conventional phased arrays with RF combining networks have fast scanning directive beams. However, the feed network loss and mutual coupling loss in a conventional phased array tend to cancel out any benefits hoped to be achieved unless very costly alternatives, such as digital beam forming techniques, are used.
In U.S. Pat. No. 5,905,473, an adjustable array antennaxe2x80x94having a central, fixed, active, antenna element and multiple, passive, antenna elements, which are reflective (i.e., re-radiates RF energy)xe2x80x94is taught. Active control of the passive elements is provided through the use of switches and various, selectable, impedance elements. A portion of the re-radiated energy from the passive elements is picked up by the active antenna, and the phase with which the re-radiated energy is received by the active antenna is controllable.
The present invention provides an inexpensive, electronically scanned, antenna array apparatus with low loss, low cost, medium directivity, and low back-lobe, as required by high transmission speed cellular systems operating in a dense multi-path environment. The enabling technology for the invention is an electronic reflector array that works well in a densely packed array environment. The invention is suitable for any communication system that requires indoor and outdoor communication capabilities. Typically, the antenna array apparatus is used with a subscriber unit. Other than the feed network, the antenna apparatus can be any form of phased array antenna.
According to the principles of the present invention, the directive antenna includes multiple antenna elements in an antenna assemblage. A feed network connected to the antenna elements includes at least one switch to select a state of one of the antenna elements to be in an active state in response to a control signal. The other antenna elements are in a passive state, electrically coupled to an impedance to be in a reflective state. The antenna elements in the passive state are electromagnetically coupled to the selected active antenna element, allowing the antenna assemblage to directionally transmit and receive signals. In contrast to U.S. Pat. No. 5,905,473, which has at least one central, fixed, active, antenna element, the present invention selects one passive antenna element to be in an active state, receiving re-radiated energy from the antenna elements remaining in the passive state.
The directive antenna may further include an assisting switch associated with each antenna element to assist coupling the antenna elements, while in the passive state, to the respective impedances. The impedances are composed of impedance components. The impedance components include a delay line, lumped impedance, or combination thereof. The lumped impedance includes inductive or capacitive elements.
In the case of a single switch in the feed network, the switch is preferably a solid state switch or a micro-electro machined switch (MEMS).
The antenna assemblage may be circular for a 360xc2x0 discrete scan in N directions, where N is the number of antenna elements. At least one antenna element may be a sub-assemblage of antenna elements. The antenna elements may also be telescoping antenna elements and/or have adjustable radial widths. The passive antenna elements may also be adjustable in distance from the active antenna elements.
The impedance to which the antenna elements are coupled in the passive state are typically selectable from among plural impedances. A selectable impedance is composed of impedance components, switchably coupled to the associated antenna element, where the impedance component includes a delay line, lumped impedance, or combination thereof. The lumped impedance may be a varactor for analog selection, or capacitor or inductor for predetermined values of impedance.
The directive antenna is suitable for use in a high data rate network having greater than 50 kbits per second data transfer rates. The high data rate network may use CDMA2000, 1eV-DO, 1Extreme, or other such protocol.